Do you want to understand a little more about the ground reaction forces associated with running? How about a little up-skilling on wearable resistance (WR), injury, rotational overload and movement specific strength training? If so, then step inside this article and I’ll run through this with you.
Some of you out there might think that introducing wearable resistance (WR) to your running with either trunk, arm, thigh or calf loading, increases the potential for injury, because you are adding load to the body so the forces you experience at foot strike are going to be greater. Well let me allay your concerns and in fact in a later article I will suggest how you can use WR to improve your injury resistance to common running injuries.
Before I get sidetracked let’s go back to this initial concern of the additional weight potentially causing greater impact forces at foot strike and therefore heightening the chance of injury. These forces at foot strike are called ground reaction forces (GRF) and are measured by force plates that are embedded in the ground (Figure 1A) or within a treadmill Figure 1B).
These force plates measure GRF in three planes of motion – medio-lateral, anterior-posterior (horizontal) and vertical as shown in Figure 2. As you can see from the diagram the vertical GRFs (vGRF) are by far the biggest forces and the ones we are concerned with in terms of adding load and causing injury. These vertical forces are a function of how much you weigh and the acceleration due to gravity. If you weigh more you will have a greater vGRF and if you (or more specifically your centre of mass – which is near your navel) drop from a higher distance the effects of gravity will have a greater influence on these forces.
So theoretically with the addition of WR you have additional mass and therefore greater vGRFs. However, this is not the case and there is research that has shown the addition of quite substantial masses (e.g.10% body mass) there has been a reduction in the vGRF (1), (2). How can this be? Remember your vGRF is made up of two components, (body mass and acceleration due to gravity) even though we are increasing the mass of the runner, the rise and fall of their centre of mass is less as shown in Figure 3. The runner’s centre of mass on the right does not rise as high because of the vest loading, and therefore the effects of acceleration due to gravity are less.
Let me share another couple of insights as why you don’t need to worry about these vGRF. First, the dominant loading we use with runners is calf loading as shown in Figure 4. We usually use loads of 1 to 2% body mass on the calf so if you weigh 70 kg then that is between 700 to 1400 grams of loading, which equals 350 to 700 gms on each calf sleeve. This is very light loading and so therefore not a lot of additional mass on the runner and hence not a great change in the vGRF. The real game changer with limb loading however, is that the overload we are exposing the muscles to is a rotational overload (see Figure 5) and therefore the effects on vGRF will be minimal.
I hope this article helps to provide context and a more in-depth understanding around why using WR limb loading has minimal effects on your vGRF at foot strike. The take home message is that limb loading is a rotational overload that has fascinating implications for the strengthening of the muscles around the hip and knee. In fact, you can strength train while you run. I believe that WR has an important role in preventing or improving resistance to common running injuries, however, that is another story to share another day. Until then slap those calf sleeves on and enjoy the benefits of resisted running.
(1) Cross, MR, et al(2014). Effects of vest loading on sprint kinetics and kinematics. Journal Strength Conditioning Research 28(7): 1867–1874.
doi: 10.1519/JSC.0000000000000354
(2 ) Couture et al (2018) Effects of upper and lower body wearable resistance onspatio-temporal and kinetic parameters during running. Sports Biomechanics, doi.org/10.1080/14763141.2018.1508490
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